Radio receiver system



RADIO RECEIVER SYSTEM Original Filed Nov. 29, 1947 AAAAA TRIGGER PU LS EGENERATOR FIG. I

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l V 69 TO VACU M 40 TUBE EDWIN E TURNER JR VOLTMETER BY FIG. 2 1 ms A oNEY Patented Jan. 26, 1954 RADIO RECEIVER SYSTEM Edwin E. Turner, In,West Roxbury, Mass, as-

signcr to Raytheon Manufacturing Company, a corporation of DelawareOriginal application November 29, 1947, Serial No. 788,811, new PatentNo. 2,496,900, dated February 7, 1950. Divided and this applicationApril 26, 1949, Serial No. 89,603

3 Claims.

This is a division of my copending application, Serial No. 788,811,filed November 29, 1947, now Patent No. 2,496,900, dated February 7,1950.

The present invention relates generally to radio echo detecting andranging systems, and more particularly to a system having very highranging accuracy and very low minimum range. The invention isparticularly suited for use in altimeter and collision warning systemsin aircraft.

A particular object of the present invention is to provide a novel radioreceiver antenna input circuit which is particularly suited to thereception of short pulse signals.

An absolute radio altimeter which will constantly indicate with a highdegree of accuracy the height above ground, even to low values of theorder of ten feet, of a fast-moving aircraft at-all times and with theutmost simplicity has been sought after for some time. Such a systemrequires the employment of extremely short transmitted pulses, andconsequently the band width of the received system must be extremelygreat. While radio echo altimeter systems have heretofore been devised,they have usually been lacking in high ranging accuracy and low minimumrange, and have been heavy and complex in structure. The presentinvention provides a system which possesses low minimum range with highranging accuracy, and may be made light in weight and relatively simplein construction, and which provides the desired range information as anindication on a conventional direct-reading galvanometer type meterwhich may be located where desired. As additional features, the metermay be adjusted to read range or height accurately from within thesystem, providing a reliable and simple performance check.

The invention envisions the employment of a very simple spark-typetransmitter which provides exceedingly short pulses of electromagneticradiation, of the order of 0.01 microsecond. These pulses are generated.and radiated through a single unitary instrumentality. There is providedalso an equally simple receiving, detecting, and amplifying systememploying a separate antenna which incorporates a detector withinitself, and, as an amplifier, only one electron tube of the electronmultiplier type is provided. The receiver system is not tuned and henceis responsive to an exceptionally wide band of frequencies. In additionit requires little or no servicing. Range information is constantlyprovided on a meter through a memory device which operates at lowvoltages and is actuated. only to record changes in the condition beingobserved. The memory device is arranged to perform ascurately, and isnot adversely affected by the in dicating device connected thereto.

The invention is described below in connection with the drawing, whichshows schematically in Fig. 1 an apparatus that may be used as anabsolute altimeter or as a collision warning device, and in Fig. 2 amodification to provide for calibration of the meter.

In the system shown in Fig. 1, pulsed electromagnetic radiation isgenerated and propagated into space by a pulse generator-transmitter 80,shown generally in a dotted-line box. Pulse generator-transmitterssuitable for the purposes of the present invention are shown in variousforms in Patents Nos. 2,407,245; 2,408,405; 2,408,406; and 2,417,052.The pulse generatortransmitter 10 shown in Fig. 1 is generally like Fig.2 of Patent No. 2,417,052, and will not be further described except toidentify its principal parts, which are a spark gap ll, capacitormembers l2 thereto connected, dipole radiators IS, a reflector It, and ahigh tension step-up transformer 15. A storage capacitor it provides theenergy for spark discharges. A battery ll provides current for chargingthe capacitor it, the charging path being completed through a protectiveresistor 18, ground, and the primary winding 2| of the transformer I5. Anormally nonconductive electronic switch tube It, which is preferably agaseous discharge tube, and may desirably be of the hydrogen thyratrontype, discharges the capacitor IE to ground when rendered conductive, toprovide a current surge in the primary winding M of the transformer It.This surge provides a high voltage in the secondary winding 22, and apulse of electromagnetic energy is generated, as described in theabove-mentioned patents. The resistor l8 prevents short-circuiting ofthe battery I! during conductivity of the tube I9.

A trigger pulse generator 23 provides a very short positive voltagepulse 24 which is fed to the switch tube l9 and to a saw-tooth voltagegenerator 25 in parallel. The positive pulse 24 is applied to thecontrol grid of the switch tube i0 through a coupling capacitor 26. Thisgrid is biased negatively with respect to the cathode by means of abattery 2?, which is connected between the grid and the cathode througha current-limiting resistor 28. The positive charge stored in thecapacitor 25 after a positiv pulse 25 has been applied to the grid leaksoff through the resistor 28, ground, and the ground connection wire 29of the pulse generator 23, so that the tube l9 soon again becomesnon-conductive in preparation for the next trigger pulse. The dischargepath of the storage capacitor i0 is through the tube 19, the primarywinding i i of a transformer .83 (the purpose of which will be explainedbelow), ground, and the primary winding 2| of the pulse transformer IS.The impedance of this path is very low, so that the capacitor i6discharges very quickly and the tube [9 is therefore quicklyextinguished. This arrangement permits the employment of high pulserepetition rates, of the order of 300,000 pulses per second, which areuseful in short range systems for altimeter purposes.

The saw-tooth voltage generator 25 is of a conventional form, andcomprises an electron tube 30, which may be of the gaseous type, acharging capacitor 3| wherein a substantially linearly increasingvoltage is periodically generated, and a charging resistor 32. Thecharging capacitor 3! and charging resistor 32 are connected in seriesacross a source of unidirectional voltage. 33+ to ground, and the tube30 is connected at its anode and cathode across the capacitor 3|. Aresistor 33 is provided in the cathode circuit, through which thecharging current flows and biases the cathode positively with respect togro nd. The charging capacitor 3| is charged only to a small fraction ofthe available 13+ voltage at any time, so that the voltage across thecapac tor increases substantially l nearly with time during eachcharging o eration. The control electrode 36 of the tube 30 is connectedto ground through a resistor 34, and to the cathode through theresistors 33 and 34 in series. and is thus biased negatively with repect to the cathode. The bias provided is suflicient to maintain thetube 30 non-conductive for the ran e of anode-cathode potentials that isused. The positive pulse 24 is applied to the control electrode 35 throuh a cou ling capacitor 35, and renders the tube 30 conductive at aboutthe same time that the enerator-transmitter switch tube I9 is fired. Thepositive volta e that is thus a p ied to the contro electrode 3"i leaksofi" throu h the grid resistor 34, ground, and the tri er pulse groundwire connection as and. due to the low anode-cathode volta e that ex stswh n the char ing capacitor 3! is discha ed through the tube, the tube30 is very ouirkiv rendered non-conductive a ain. Rechar ing of thecapacitor 3! provides the positi e cathode (or ne at ve grid) biasthrough the cathode resistor 33 to maintain the tube nonconductive untilthe next succeeding trigger pulse 24.

At the term nation of the conductive period of the tube 30, the nextsucceeding linear volta e sweep commences, almost s multaneously withthe generation and transmission of a ulse of rad o frecuencv ener y bythe generatortransmitter i i. The d schar e time of the sawtoothgenerator capacitor 31 through the tube 3 may introduce some delay inthe commencement of the sweep volta e. To avoid this delay and start thesweep voltage exactly when the en.- er y pulse is transm tted, a haseshifter 31 may be interposed between the trigger pulse generator 23 andthe switch tube iiiif desired, so that the trigger pulse 24 fed to theswitch tube will arrive there a small time after the saw-tooth voltageenerator tube 30 is fired.

The saw-tooth volta e generated n the saw tooth generator ca acitor 3|is applied across a measuring capacitor 40 through a normally-openelectron c switch t and round. The normallyopen switch is controlled bya radio receiver, as will be described below.

The radio receiver system has an antenna 42, wh ch may incl de as itselements a di ole 43 and a pa ab l c reflector as. A un directionalconductor 45, for example, a crystal, is connected directly between theelements of the dipole 4,3 as

45 a detector, so that pulsed unidirectional current only appears in theantenna connection cable 46 when echoes of the transmitted energy arereceived in the receiver antenna 32. To protect the crystal 55 frombeing burned out during the generation and transmission of energypulses, the generator-transmitter It and the receiver antenna 42 may belocated at opposite ends of the wing of the aircraft.

Unidirectional current pulses from the receiving antenna 42 are broughtto a wide-band amplifier which comprises an electron-multiplier tube 50having a photoemissive cathode 5!, a control electrode 52, a pluralityof secondary electron emitters 53, an anode and a light source 55. Thelight source 55 is energized by a battery 56, and arranged to illuminatethe cathode 5 which emits electrons. A battery 5! provides positivepotentials for the secondary electron emitters 53 and the anode 54. Thenegative terminal of the battery 5'! and the cathode 5| are grounded,while the positive terminal of the battery is connected to the anode 5dthrough a plate load resistor 64. Each of the secondary elec tronemitters 53 is connected to an individual intermediate point on thebattery 5? through a current-limiting resistor 58. The secondaryelectron emitters 53 are each made increasingly positive with relationto the cathode 5!, the most positive secondary electron emitter beingthe one furthest from the cathode 5!, or nearest to the anode 54.Electrons emitted from the cathode are thus attracted to the secondaryemitter 53 nearest to the cathode, where by im act they cause theemission of an increased number of secondary electrons. The secondaryelectrons are further attracted to the next furthest removed secondaryemitter, where the process is repeated, until the electron stream thatarrives at the anode 5 is vastly multiplied in density by comparisonwith the electron stream emitted by the cathode.

The particular electron multiplier tube employed may have aphotoemissive cathode and a light source, as shown and described herein,or, if desired, a thermionic cathode may be employed. Either kind ofcathode can furnish electrons by random emission. However, thethermionic cathode requires heat and consequently eventually causescontamination of the secondary emitter surfaces. In the practicalconstructions of photoemissive electron multipliers that are available,the light source is either enclosed, or the light is brought to thecathode by a devious path by way of a mirror, so that no contaminationof the secondary emitter surfaces occurs. Due to these advantageouscharacteristics, the photoemissive electron multiplier provides a veryhighly satisfactory wide band amplifier, having a high signal-to-noiseratio.

The flow of electrons fromthe cathode 5! is controlled by a control grid52, interposed between the cathode and the nearest secondary emitter 53.The control grid 52 is normally biased negatively with respect to thecathode 5! by means of a battery 59 connected between the cathode andthe control grid throu h a resistor BI. The electron stream is thus heldto a certain low, steady-state value, or, if desired, out 01faltogether. The unidirectional signals from the antenna 42 are appliedacross the grid r sistor 6| through a ca acitor 62 in such a fash ion asto drive the control grid 52 in a positive direction with relation tothe cathode 5!. A resistor 63 of high value is connected across theunidirectional conductor 45, and the charge thus given to the controlgrid 52 leaks ofi through this resistor, through a circuitincluding thebias battery 59 and the. grid resistor iii. The time constant or" thiscircuit is preferablyv chosen to be very short. where'the electrondensity is low, and thereafter amplification takes place as acharacteristic of the tube to. itself. The electron strearn'thusincreases in density when an echo pulse is received in the antenna 412,and the resultant current flow in the plate load resistor 55 causes avoltage drop at the anode 5 which is employed to close the normally openswitch t! to place a charge on the measuring capacitor lit in a mannerthat will now be described.

The normally open switch it comprises a pair of triode sets, the firstset comprising a first anode 65, first cathode St, and first controlelectrode 5'5, and the second set comprising a second anode ll, secondcathode l2, and second control electrode l3. The first anode and thesecond cathode ii are connected together and to the output of thesaw-tooth generator 25. The first cathode and second anode ii areconnected together and to one side of the measuring capacitor The otherside of the measuring capacitor is grounded. The first control electrode5! is connected to the first cathode 53 through the secondary winding 53of a first pulse transformer '58 and a first bias battery 59 in series.The second control electrode :3 is connected to the second cathode 52through the secondary winding it of a. second pulse transformer Hand 2.second bias battery 75 in series. The bias batteries es and it normallybias the triode sets in which they are connected to cutoff, thus keepingthe switch dl normally open.

The voltage drop at the electron multiplier anode 54 is the receivercutout signal, in the form of a unidirectional output pulse, and isconnected through a coupling capacitor '56 to the primary rindings 85and 82 of the first and second pulse transformers l3 and 79 in series.The connecticn is such that, when there is a receiver output signal,both control electrodes 5? and T3 are driven sufliciently positive withrelation to their respective cathodes 6% and i2 to overcome the biasfurnished by the batteries 59 and I5, respectively. The transformer 33is employed as a lock-out pulse transformer and has its primary winding8:1 connected in series in the anodecathode circuit of thegenerator-transformer switch tube l9, and its secondary winding 85connected in series in the signal transmitting circuit to thenormally-open switch M. The signal transmitting circuit thus includes,in all, in

series connection, the coupling capacitor is, the secondary winding 35of the look-out transformer, the primary windings 8! and 82 of the twopulse transfoirners, ground, the amplifier battery 5?, and the outputresistor 84 in which the output signal appears. [my charge remaining onthe coupling capacitor it after the transmission of a pulse therethroughis dissipated through the signal transmitting circuit. The time constantof this circuit is preferably very short.

The output pulse thus furnished to the normally-open switch ii overcomesthe negative bias of each triode section. The triode sections arereversely connected in parallel in the circuit to the measuringcapacitor fi, so that one or the other triode section conducts currentdepending on which of the two capacitors 3| and Mthas the higherpotential at the time-when the" output pulse is impressed on the switch4!. Thus, if the saw-tooth generator capacitor 3! is at that The gridcontrol is eitective' instant charged" more positively than the Incasuring' capacitor 40, the'first anode 65 and first cathode? 66 becomeconductive; or, if the sawtooth generator capacitor 35 is at thatinstant charged only. to a-value that is negative with respect to' thepotential of the measuring capacitor "40, thesecond anode H and secondcathode: T2 becomeconductive. In either event, the measuring capacitor401s momentarily connected to the saw-tooth generator capacitor 35 andassumes the same charge, or, more exactly, the same potential levelabove'ground. The closing of the'switch ll is momentary only, becausethe pulse transformer-sunblock their respective control electrodes onlywhen there is a sudden voltage change in the plate load resistor to.This sudden change is insensitive to the duration of the pulse thatfollows. The lock-out pulse transformer iscOnnectedin the signaltransmitting circuit-ina-fashion to oppose the action of the two signalpulse transformers S! and 52, and is effective to prevent unblocking ofthe control electrodes 61 and l3 during the generation and transmissionof an-energy pulse. Any grid current that may be drawn by the controlelectrodes 6'5 and 13 is confined to the respective local gridcathode,and does not afiect the charge on the measuringcapacitor lil. Such gridcurrent, if it does occur,- may be minimized by the inclusion of gridcircuit resistors (not shown) in the respective grid-cathode connectionsif desired.

The potential that is present at any time on the measuringcapacitorsB-is indicated-in a vacuumtube voltmeter circuit comprisingtwo electrontubes 9| and 92. The first tube 8! is connected atits anode 93 directlyto a source of unidirectional voltage B-[-, which is preferably wellregulated, and at its cathode at. to ground through a cathode resistor95. The control electrode 96--is-connected to the switch side of themeasuring. capacitor 49. The second tube 92 is connected at its anode 9?to the'B-isupply, and at its cathode 98 to ground through a secondcathode resistor 99. A potential divider lot is connected directly fromB+ to ground, and the second tube control electrode N32 is connected tothe potential divider through a variable tap 83. A meter Ill-4, whichmay be a sensitive galvanometer, and a variable resistor Hi5 areconnected in series between the two cathodes 94 and. 98.

The first'tub'e SI of the vacuum tube voltmeter is'oper'a'te'd as acathode follower, and thus has substantially infinite impedance, andnever draws grid current. Thecathode potential follows the gridpotential very accurately, and the charge on the measuringcapacitor'cannot be dissipated through the tube. The second tube isoperated as a voltage divider; and, by means of the adjustabletap' I03,adjusts the potential of the secondtubecathode 98 to be the same as thatof the first tube cathode when the meter MM is expected to read zero.The adjustable tap N33 is thus a" miniinum'reading adjustment. Thevariableresist'or I05 controls the amount of meter deflection pervoltage unit, and adjusts the meter to readadesired maximum value. Thesetwo adjustments are the range calibration adjustments of'the'system,and, as will be shown, can be made within thesystem.

In the system of the invention described above, the measuring capacitorMl functions as a memory device" withv respect to the range or distanceto. a} single. target that is-being' continuously observed, for example,the earth be- 'as the sweep capacitor.

tor or sweep capacitor 3! and the measuring capacitor 46 together islow, only a few signals need be received by the receiver to charge themeasuring capacitor to the same potential level Thereafter if the range,or in this case, the altitude, is increasing, each received signal willbe effective to render the switch 4.! conductive through the first anode65 and first cathode 66, for the sweep voltage increases with time. Onthe other hand, if the altitude is decreasing, the second anode H andsecond cathode 72 will become conductive during each observationthereof, for the measuring oapacitor 40 will then be charged morepositively than the sweep capacitor (N. If there is no change in thealtitude between succeeding received echoes, neither triode section ofthe switch 4| will become conductive, for, although the controlelectrodes 6'! and '53 will be momentarily disposed to permitconduction, the respective anodes and cathodes will all be at the samepotential, and no electrons will pass through either triode section. Asmentioned above, if grid current should occur, it is localized in therespective grid-cathode circuits, and does not effect the charge on themeasuring capaciton' Since the vacuum tube voltmeter has practicallyinfinite input impedance, the memory of the measuring capacitor 40 isvery long; that is, the capacitor holds its charge for a very long timein the absence of signals that alter it in the normal operation of thesystem. Alteration of the charge on the measuring capacitor is made ineither direction, so that there is no delay in responding to changes interrain contour or range to an object in horizontally directed measuringsystems. Since, as mentioned above, the circuit to the measuringcapacitor 40 has little resistance, response to sudden changes in rangeor terrain contour is accurate. The meter H14 reads the range oraltitude directly.

When the sweep length is known in terms ofrange or distance, and thesweep is synchronized with the transmitted pulse, the meter I04 can becalibrated within the. system by means of the modification shown in Fig.2. To the measuring capacitor switch All are added two single-poledouble throw switches ill and H2. The first switch H! is a Minimum, orZero Range, ad- J'ustment switch. Normally this switch connects thesecond control electrode l3 to the second cathode 72 as shown in Fig. 1,but for calibration purposes this grid is connected directly to thesecond anode H. The second triode section is then rendered conductiveover the whole sweep cycle of the saw-tooth generator 25, and themeasuring capacitor 40 assumes the lowest sweep voltage available. Inthe vacuum tube voltmeter, the Minimum adjustment tap I03 is thenadjusted until the voltage difierence between the two cathodes 94 and 98is zero, and the meter H34 reads zero. The first switch III is nowreturned to the normal position, and the second switch H2 is operated tothe calibrate position.

This has the effect of making the first triode section conductivebetween the first anode 65 and first cathode 66 over an entire sweepvoltage cycle, and the measuring capacitor 40 assumes the highest sweepvoltage available. In the vacuum tube voltmeter, the Maximumadjustmentresistor l is then adjusted until themeter reads the maximumrange to which the full sweep amplifier having its input length isequivalent. The second switch H2 is then returned to its normal positionto connect the first control electrcuie 61 to its cathode 66 and restorenormal operation of the measuring capacitor switch 4|.

Many modifications of the above described embodiment may be made withinthe scope of the invention to accommodate a particular employmentthereof. For example, the meter I94 may be replaced by any one of manyforms of threshold alarms known to the art, and the invention can thenserve to light a lamp or sound a bell when the altitude of the aircraftbecomes dangerously low or a collision becomes imminent. In certain ofits aspects, for example, the system for controlling the measuringcapacitor switch M for the measuring capacitor 40, the invention mayfind employment in other types of echo ranging equipment.

Having now described my invention, I claim: 7 1. A receiver forelectromagnetic radiation comprising: a dipole antenna; a unidirectionalconductor connected directly across the output of said antenna; and wideband amplifier means coupled through a capacitor to said unidirectionalconductor to amplify signals therefrom, said am plifierincludingelectrontube means having an input control electrode and a cathode; aresistor connected in shunt with said unidirectional conductor; acapacitive connection from one end of said resistor to said controlelectrode; and a connection from the other end of said resistor to saidcathode.

2. A receiver for electromagnetic radiation comprising: a dipoleantenna; a unidirectional conductor connected directly across the outputof said antenna; and amplifier means coupled through a capacitor to saidunidirectional conductor to amplify signals therefrom, said amplifiermeans comprising a photoemissive cathode, a control electrode, aplurality of secondary electron emissive electrodes, an anode, and meansfor illuminating said photoemissive cathode, and

ode, and a resistor connected in shunt withsaid unidirectionalconductor. a

3. A receiver for electromagnetic radiation comprising a dipole antenna,a unidirectional conductor connected directly across the output of saidantenna, a resistor connected directly in shunt with said unidirectionalconductor, and an connected across said resistor. EDWIN E. TURNER, JR.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,097,514 Chaflee Nov. 2, 1937 2,124,533 Barbulesco July 26,1938 2,131,042 Halstead Sept. 27, 1933 2,169,358 Hollmann Aug. 15, 19392,298,960 MoRae Oct. 13, 1942 2,416,376 Cawein Feb. 25, 1947 2,523,398Southworth Sept. 26, 1950 2,539,594 Rines Jan. 30, 1951 2,543,035Willoughby Feb. 27, 1951

